Carrier-gas flow

The technique just described requires the porous medium to be sealed in a cell, so It cannot be used with pellets of irregular shape or granular material. For such materials an alternative technique Introduced by Eberly [64] is attractive. In Eberly s method the porous pellets or granules are packed into a tube through which the carrier gas flows steadily. A sharp pulse of tracer gas is then injected at the entry to the tube, and Its transit time through the tube and spreading at the exit are observed. A "chromatographic" system of this sort is very attractive to the experimenter,... 

A variable-size simplex optimization of a gas chromatographic separation using oven temperature and carrier gas flow rate as factors is described in this experiment. 

The transfer efficiencies for ultrasonic nebulizers (USN) are about 20% at a sample uptake of about 1 ml/min. Almost 100% transfer efficiency can be attained at lower sample uptakes of about 5-20 pl/min. With ultrasonic nebulizers, carrier gas flows to the plasma flame can be lower than for pneumatic nebulizers because they transfer sample at a much higher rate. Furthermore, reduction in the carrier-gas flow means that the sample remains in the mass measurement system for a longer period of time which provides much better detection limits. 

Quantitative analysis using the internal standard method. The height and area of chromatographic peaks are affected not only by the amount of sample but also by fluctuations of the carrier gas flow rate, the column and detector temperatures, etc., i.e. by variations of those factors which influence the sensitivity and response of the detector. The effect of such variations can be eliminated by use of the internal standard method in which a known amount of a reference substance is added to the sample to be analysed before injection into the column. The requirements for an effective internal standard (Section 4.5) may be summarised as follows ... 

Van Deemter equation An equation relating efficiency (HEPT in mm) to linear flow velocity in a chromatographic column. The efficiency is expressed as the height equivalent to a theoretical plate HEPT = A + BIV + Cv), where A, B, and Cv are constants and V is the linear velocity of the carrier gas. This equation tells us that to obtain maximum efficiency, the carrier gas flow must be optimized. 

The purge activation time (or the sample transfer time) depends on the sample solvent and carrier gas flow relative to the volume of the injection port liner and the boiling points of the sample components. For most applications, a purge activation time of 50-120 sec is better than 25-50 sec. Early activation results in the loss of sample, while late activation results in peak tailing. A more accurate method of determining purge activation time is to divide the volume of the injector liner by the flow rate (F) of the carrier gas and multiply this value by 1.5 or 2.0. (Do not use a packed liner.)... 

Gas chromatography on a 200 cm. by 0.6 cm. column packed with 10% Apiezon L on Chromosorb W (AW, DMCS) using a flame-detector instrument, at a 40 ml./minute helium carrier gas flow rate, gives a trace peak at 9.9 minutes (diphenylmethane), a major peak at 11.7 minutes (1,1-diphenylethane), and a trace peak at 15.4 minutes (1.1-diphenylethanol) when the oven is held at 190° for 10 minutes and then programmed at 10°/minute to 290°. 

The checkers found that gas chromatographic analysis of one sample using a 305 cm. by 0.3 cm. column packed with 10% SF-96 on Chromosorb P operated at 70° with a 60 ml./minute helium carrier gas flow rate gave five minor impurity peaks, two at shorter retention times, and three at longer retention times. None of these impurities was present in greater than 1.1% total impurities wrere 3%. 

The checkers found considerable variation in the rate of the reaction in different runs, the time required for its completion ranging from 3 to 10 hours. It is therefore advisable to monitor the progress of the reaction. For this purpose small aliquots (ca. 0.05 ml.) were withdrawn from the flask with a syringe and hydrolyzed by injection into a vial containing ether and saturated ammonium chloride. The relative amoimts of enol silane and cyclopropoxy sdane were determined by gas chromatography on an 0.6 cm. X 3.7 m. column of 3% OV-17 coated on 100-120 mesh Chromosorb W. With a column temperature of 120° and a carrier gas flow rate of 20 ml. per minute, the retention times for the enol silane and the cyclopropoxy silane are ca. 1.9 and 2.3 minutes, respectively. 

Figure 2, GC-TEA (N mode) chromatogram of a tire factory air sample. The column was a 5,5 m glass tube, 2mm, i,d, packed with Carbopak B(4% Carbowax 20 M, with 0,8% KOH on charcoal). Carrier gas flow rate was 15 mL/min, Column temperature was held at 40°C for 2 min, and then increased by 8°/min to 180°C, Peak identity 1-dimethylamine, 2-trimethylamine, 3-diethylamine, 4-tri-ethylamine, and 5-morpholine,...

For the AOAC beer samples, a 2 m x 2 mm glass column packed with 8.57o Carbowax 20 M + 0.857, NaOH on 100/120 mesh Chromosorb G was used at 130 C and a helium flow rate of 20 cc/min. Retention times of NDMA and NDPA were 4.5 and 12.2 min, respectively. For the ASBC collaborative study, a 1 m x 2 mm glass column containing 67, Carbowax 20 M-TPA on 100/120 mesh Chromosorb G was operated at 90 C with 20 cc/min helium flow rate. Retention times were 3.6 and 11.3 min for NDMA and NDPA, respectively. For determination of nitrosamines in amines, a 2 m X 2 mm, 107, Carbowax 20 M-TPA on 100/120 mesh Chromosorb G column was operated at 190 C with a carrier gas flow rate of 20 cc/min. Retention times were NPYR, 6.6 min NMOR, 7.4 min. 

The GC/FID conditions were as follows column, 1.5% OV-17 (2 m x 3-mm i.d.) glass column N2 carrier gas flow rate, 45mLmin temperature of injection port, column and detector, 240,235 and 235 °C, respectively. The recoveries of these amino derivatives with fortification level ranging from 0.5 to lO.Omgkg" were 62-101% for chlornitrofen, 62-101% for nitrofen and 58-101% for chlomethoxyfen, and satisfactory recoveries from soil were obtained at high concentrations, but the recoveries at lower concentration averaged about 66% for the least recovered compound. Interference from other substances in the soil extracts derived from the acetylation reaction was negligible. 

Gas chromatograph Injection port Temperature Carrier gas Flow rate Column... 

Increasing the speed of analysis has always been an important goal for GC separations. All other parameters being equal, the time of GC separations can be decreased in a number of ways (1) shorten the column (2) increase the carrier gas flow rate (3) reduce the column film thickness (4) reduce the carrier gas viscosity (5) increase the column diameter and/or (6) heat the column more quickly. The trade-off for increased speed, however, is reduced sample capacity, higher detection limits, and/or worse separation efficiency. 

Injection volume Carrier gas flow rate Transfer line temperature Ionization mode Detector calibration Acquisition type Acquisition masses... 

The separation nuaber is the only column efficiency par2uaeter that can be deterained under teaperature progr2uued conditions [45,46]. The critical parameters that aust be standardized to obtain reproducible SM values for coluans of different length are the carrier gas flow rate and the temperature program. The SN is widely used as part of a standardized test method to evaluate the quality of open tubular columns for gas chromatography (section 2.4.3). 

Mathematical methods for determining the gas holdup tine are based on the linearity of the plot of adjusted retention time against carbon number for a homologous series of compounds. Large errors in this case can arise from the anomalous behavior of early members of the homologous series (deviation from linearity in the above relationship). The accuracy with which the gas holdup time is determined by using only well retained members of a homologous series can be compromised by instability in the column temperature and carrier gas flow rate [353,357]. The most accurate estimates... 

Figure 8.26(A) is an example of a valve type interface [329]. Helium carrier gas is provided to the headspace saiq)ler and is split into two flow paths. One path is flow-controlled and provides a constant flow of carrier gas which passes from the headspace unit through the heated transfer line to the gas chromatograph. The second flow path is pressure-regulated and, in the standby mode, the seunple loop and seuapling needle are flushed continuously by the helium flow. At a time determined by the operator, the sampling needle pierces the septum and helium pressurizes the headspace vial to any desired pressure. The headspace gas is then allowed to vent through the sample loop. Once filled, the sample loop is placed in series with the normal carrier gas flow and its contents are driv Bbhrough the heated...

In a typical experimental arrangement, the injection block heater of the gas chromatograph is used to heat a short catalyst bed containing platinum, palladium, copper or nickel coated on a diatomaceous support. The catalyst bed can be the top portion of a packed column or a precolumn connected to a packed or open tubular column. Hydrogen carrier gas flows through the heated catalyst bed (220-350 0) and then into the column. The sam B is injected by... 

Normal alkanes or fatty acid methyl esters are generally used as the standard homologous compounds. The column separation number is dependent on the nature of the stationary phase, the column length, column temperature, and carrier gas flow rate [42-44]. Referring to Figure 1.2, at a sufficiently high capacity factor value either n, N, or SN provides a reasonable value for comparing... 

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